Some common terms you may see when looking at open access publications:
Green OA: This is the "self-archiving" route to OA. Depending on the contract terms you have signed with your publisher, you as an author of a scholarly publication can deposit a copy of that publication into an open access repository. In addition to publication in a traditional scholarly journal, you may be able to deposit a pre-print or post-print copy of your article into an open access repository or on your personal website.
Gold OA: This is publisher-level open access. You write a paper and submit it to a journal in the usual way, and the publisher makes the article freely available upon publication with no additional effort on your part. Some journals are entirely OA (they do this for every article) and some publish under a hybrid model (some articles in an issue will be available OA and some will not). Some OA publishers will charge an Article Processing Charge (APC) in order to publish an article OA - this fee is often paid by a funding institution or a university, but is sometimes paid directly by the author. (Note: Not every publisher that charges an APC is predatory, but some may be. A list of resources about predatory publishing can be found here: Predatory Journals WebQuest.
Gratis OA: Gratis OA means open access materials that are free to read, but that still have copyright/licensing restrictions. This means you will need to investigate further to find out if you are able to re-use, re-mix, or re-distribute the work, and that usually these rights will be restricted. You may also see this referred to as "weak OA."
Libre OA: Libre OA are open access materials that are free not only to read but also to use. These are materials that you can freely photocopy and share with your students in class, for example. (Note that you cannot always legally do this with Gratis OA materials or library subscription materials!) These materials are typically free of most copyright and licensing restrictions. You may also see this referred to as "strong OA."